Mobility Lifetime Research Articles

Metal nanocrystal memories-part II: electrical characteristics

This paper describes the electrical characteristics of the metal nanocrystal memory devices continued from the previous paper [see ibid., vol. 49, p. 1606-13, Sept. 2002]. Devices with Au, Ag, and Pt nanocrystals working in the F-N tunneling regime have been investigated and compared with Si nanocrystal memory devices. With hot-carrier injection such as the programming mechanism, retention time up to 10/sup 6/ s has been observed and 2-bit-per-cell storage capability has been demonstrated and analyzed. The concern of the possible metal contamination is also addressed by current-voltage (I-V) and capacitance-voltage (C-V) characterizations. The extracted inversion layer mobility and minority carrier lifetime suggest that the substrate is free from metal contamination with continuous operations.

CdTe and CdZnTe detectors for timing measurements

We measured timing properties of CdTe and CdZnTe semiconductor detectors with planar configuration. We developed a new method to evaluate their performance in timing resolution utilizing an /sup 241/Am-doped plastic scintillator. We confirmed that the low mobility and short lifetime of holes are major obstacles to their timing resolution. However, their timing properties can be very much improved, either by applying a high electric field that increases the carrier speed or by selecting those events which are dominated by the electron signal. We demonstrated the latter through a pulse-shape discrimination technique using two different integration time constants. In conjunction with a newly developed CdTe diode, we obtained a superior timing resolution of 5.8 ns. We also discussed the application of CdTe to positron emission tomography (PET), employing the standard 511 keV gamma-gamma coincidence method. We confirmed that a geometrical configuration in which the electrodes are parallel to the incident /spl gamma/-rays gives about three times better timing response than a geometry when the electrodes are perpendicular to the /spl gamma/-ray beam.

CdTe stacked detectors for gamma-ray detection

We describe a stacked detector made of thin cadmium telluride (CdTe) diode detectors. By using a thin CdTe device, we can overcome the charge loss problem due to the small mobility and short lifetime of holes in CdTe or cadmium zinc telluride (CdZnTe) detectors. However, a CdTe detector with a thickness of more than 5 mm is needed for adequate detection efficiency for gamma-rays of several hundred keV. Good energy resolution and good peak detection efficiency are difficult to obtain using such a thick CdTe detector. The stacked detector enabled us to realize a detector with both high-energy resolution and good efficiencies for gamma rays up to several hundred keV. In order to verify this concept, we constructed a prototype made of ten layers of a 0.5-mm-thick CdTe diode detectors with a surface area of 21.5 mm/spl times/21.5 mm. With this, we have achieved 5.3-keV and 7.9-keV energy resolution [full width at half maximum (FWHM)] at 356 keV and 662 keV, respectively, at the temperature of -20/spl deg/C.

Morphology and electronic transport of polycrystalline silicon films deposited by SiF4/H2 at a substrate temperature of 200 °C

Undoped and phosphorous doped polycrystalline silicon (poly-Si) films were deposited using a SiF4/H2 gas mixture at a substrate temperature of 200 °C by radio frequency plasma enhanced chemical vapor deposition (rf-PECVD). Fourier transform infrared (FTIR) spectroscopy and x-ray diffraction (XRD) experiments reveal that the present poly-Si films are equivalent to the poly-Si films deposited at high temperature (>600 °C). XRD and scanning electron microscope observations show that the crystalline quality of slightly P-doped film is better compared to that of undoped poly-Si films. Phosphorus atom concentration in the slightly P-doped poly-Si film is 5.0×1016 atoms/cm3. Association of a few phosphorous atoms in the silicon matrix enhances crystallization as eutectic-forming metals do. Dark conductivity of slightly P-doped film is 4 orders of magnitude higher, although mobility–lifetime product (ημτ) is 2 orders of magnitude lower than that of undoped film. The presence of higher density of strained Si–Si bonds in the amorphous regions of slightly P-doped films revealed by FTIR spectroscopy reduces the value of ημτ.

X-ray induced effects in stabilized a-Se X-ray photoconductors

We report on X-ray induced changes in the electrical properties of stabilized amorphous selenium typical of the material used in direct conversion X-ray imaging devices. Carrier mobility and deep-trapping lifetime were measured using time-of-flight and interrupted-field time-of-flight (IFTOF) measurements. The hole and electron drift mobility is not affected by up to 1 R of exposure to 50 kV p X-rays. The hole lifetime decreases from 50 to 27 μs after exposure to 0.48 R. The hole lifetime slowly recovers to its initial value after circa 3000 min. The electron lifetime does not change after exposure to 1 R. The results are explained by an accumulation of excess hole traps due to capture of electrons in deep localized states. We also observe small changes in the dark current after exposure to X-rays.

Light-soaking and annealing kinetics of majority and minority carrier mobility–lifetime products in a-Si:H

A study on undoped a-Si:H of the light-soaking/annealing process of majority as well as minority carrier properties and their relations to defect density is presented. Defect density was measured by the constant photocurrent method (CPM), electron mobility–lifetime product was deduced from photoconductivity and hole mobility–lifetime product was obtained from measurements of ambipolar diffusion length by steady-state photocarrier grating technique (SSPG). We confirm the already known hysteresis behaviour in the dependence of electron mobility–lifetime product on defect density, and report a new and opposite hysteresis-like dependence of the hole mobility–lifetime product during the light-soaking/annealing process. The changes in the mobility–lifetime products of both carriers and concomitant changes in the defect density and in the Fermi level are discussed on the basis of recombination kinetics and structural order effects.

Optical radiation and temperature-dependent microwave performance of optically biased GaAs metal-semiconductor field effect transistor

A physics-based analytical model for an optically biased GaAs metal-semiconductor field effect transistor, suitable for simulating optoelectronic integrated circuits, is presented. The main objective of the model is to show the effect of optical radiation and temperature on important parameters such as the bandgap energy, mobility, and carrier lifetime. It is found that optical radiation has negligible effect on these parameters, but the temperature affects them significantly. The effect of different scattering mechanisms on the drain-source current is also studied. Using this analysis, the model extends to describe optical radiation and temperature-dependent device characteristics such as the threshold voltage, transconductance, cutoff frequency, transit time, and responsiv- ity. Under illumination, a significant enhancement in device transconductance leading to an increase in cutoff frequency is observed.

Nature of Charged Metastable Defects in Network Rebonding Model

AbstractWe recently developed an atomistic model of metastability of a-Si:H, where defect creation is driven by the breaking of weak silicon bonds. The kinetics of degradation in this model is simulated with coupled rate equations that show t1/3 kinetics of defect creation and saturation behavior similar to experiment. Saturated defect densities of neutral dangling bonds are accompanied by a much smaller density of negatively charged floating bonds and positively charged dangling bonds (D+). We propose a two-step annealing mechanism where the positively charged D+ dangling bonds are annealed at low temperature and the D0 at higher temperature -which accounts for hysteresis in mobility lifetime products.

Interstitial molecular hydrogen and deuterium in PECVD and HW amorphous silicon

For many years it has been known that high quality plasma-enhanced chemical vapor deposition (PECVD) hydrogenated amorphous silicon contains small quantities (⩽100 ppm) of molecular hydrogen in microvoids. More recently it has been demonstrated that there exist major (10–40%) molecular hydrogen populations singly trapped as interstitials in the amorphous equivalents of tetragonal T sites of the a-Si network. Comparative NMR studies have shown that a substantial fractional molecular hydrogen population residing in T sites correlates well with high photovoltaic quality as measured by film's mobility–lifetime product ημτ. Now our current NMR measurements have turned to 1H– 29Si double resonance experiments in hot wire amorphous silicon films (HW a-Si:H). Our HW a-Si:H films have been provided by Schropp and Branz and the PECVD films by Paul. The HW films show several resolved proton NMR components at 80 and 300 K. These components include a narrow line arising from molecular hydrogen resident in T-like sites and perhaps also in voids. A second broader line reflects clusters of Si-bonded H. A third superbroad (∼80 kHz FWHM) line also is present as is a diamagnetically shifted feature.

Dark conductivity, photoconductivity, and light-induced absorption in photorefractive sillenite crystals

We measure the bulk and light-induced absorption as well as the dark- and photoconductivity in doped and undoped photorefractive Bi12TiO20 crystals and compare the results obtained using different techniques. From these data we compute the quantum efficiency for the photogeneration of charge carriers and the carriers mobility–lifetime product, and characterize the suitability of these crystals for holographic recording. A two-center model theory is shown to adequately describe absorption and photoconduction in these materials.

Nanocrystalline silicon from hot-wire deposition — a photovoltaic material?

Nanocrystalline silicon (nc-Si:H) attracts a great deal of attention due to the hope for more efficient and stable solar cells, as well as better thin-film transistors and optical sensors. In this study, we report on improvements in the structural and electronic qualities of intrinsic nc-Si:H grown from hot-wire chemical vapour deposition. For examining a wide range of deposition parameters, we use a design-of-experiments approach. In contrast to our previous films obtained from tungsten and tantalum filaments, a novel type of filament greatly enhances preferential growth in the 〈110〉 direction over a wide range of deposition conditions. General considerations on the orientation and electronic activity of grain boundaries in polycrystalline silicon explain why the electronic quality of this 〈110〉-oriented film is remarkably higher than the one previously grown, mixed-phase nanocrystalline silicon. Mobility–lifetime products of films from our novel filaments are two orders of magnitude higher than those of samples from Ta wires. In photoluminescence spectra, no band tail contributions occur, and the amorphous and defect peaks are greatly reduced. Moreover, the transverse optical Raman signal is red-shifted, and thereby indicates a reduction in mechanical strain in our novel nanocrystalline silicon films.

Recent progress in CdTe and CdZnTe detectors

Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and /spl gamma/-ray detection. The high atomic number of the materials (Z/sub Cd/=48, Z/sub Te/=52) gives a high quantum efficiency in comparison with Si. The large bandgap energy (Eg/spl sim/1.5 eV) allows us to operate the detector at room temperature. However, a considerable amount of charge loss in these detectors produces a reduced energy resolution. This problem arises due to the low mobility and short lifetime of holes. Recently, significant improvements have been achieved to improve the spectral properties based on the advances in the production of crystals and in the design of electrodes. In this paper we summarize 1) advantages and disadvantages of CdTe and CdZnTe semiconductor detectors and 2) the technique for improving energy resolution and photopeak efficiencies. Applications of these imaging detectors in future hard X-ray and /spl gamma/-ray astronomy missions are briefly discussed.

Characterisation of vertical gradient freeze semi-insulating InP for use as a nuclear radiation detector

The performance of a nuclear radiation detector fabricated from Vertical Gradient Freeze (VGF) semi-insulating Fe-doped InP was investigated. Pulse height spectra were acquired when the detector was irradiated with alpha particles from 241Am, as a function of temperature and detector bias voltage. The spectroscopic performance of the detector was limited at room temperature due to the presence of a high leakage current. At a bias of −150 V, a room temperature leakage current density of 2.4×10 −6 A/mm 2 was observed which reduced to 7.1×10 −8 A/mm 2 at a temperature of −21°C. By biasing the irradiated detector contact at either a negative or positive potential, the charge collection efficiency (CCE) was measured separately for pulses produced predominantly by electron transport and for pulses produced predominantly by hole transport, respectively. At −21°C a maximum CCE of 72% was obtained for the electron signal and 44% for the hole signal. As a function of bias the CCE of the electrons remained constant in the temperature range −21°C to +19°C, whilst that of the holes exhibited a significant variation. By comparison with the Hecht relationship estimates of the carrier mobility–lifetime ( μτ) products are deduced, which are similar for both holes and electrons and in the range 5×10 −7–8×10 −7 cm 2/V. A reduction in μτ is observed at lower temperature for holes, whereas the value for electrons remains constant over the temperature range studied.

Electronic properties of CVD polycrystalline diamond films

Electronic properties of polycrystalline diamond films have been analysed using steady-state photoconductivity under UV (λ=190 nm) and red (λ=633 nm) illumination. We look at the effect of several post-deposition treatments: annealing in vacuum at 400°C, successive annealing under air and methane or the reverse, and X-ray exposure. These are shown to affect the photoresponses in different manners. Globally, the mobility–lifetime products of samples are found in the range 10 −7–10 −6 cm 2 V −1 under UV illumination, and ratios of photoresponses in UV and red are in the range 10 3–10 5. First results obtained using the modulated photocurrent technique are also presented.

On the development of compound semiconductor thallium bromide detectors for astrophysics

We discuss the detector requirements for future X-ray astrophysics missions and present preliminary results from our compound semiconductor program designed to produce X-ray detectors with high spatial and spectral resolution across the energy range of 1–200 keV. Several prototype detectors have been fabricated from monocrystalline TlBr and tested at hard X-ray wavelengths in our laboratories and at the ESRF synchrotron research facility. Energy resolutions of 1.6 keV (FWHM) at 5.9 keV and 2.6 keV (FWHM) at 26 keV have been achieved, although we find that performance is highly variable due to polarisation effects. The resolution function is dominated by high leakage current at all energies. From pulse-height measurements of 241Am as a function of detector bias, we derive the electron mobility–lifetime product at −2°C to be (2.9±0.2)×10 −4 cm 2 V −1. This is about an order of magnitude higher than previously reported values.

Characterisation and modelling of a two terminal visible/infrared photodetector based on amorphous/crystalline silicon heterostructure

In this work, we report on the detailed characterisation of a two terminal visible/infrared tuneable photodetector both in steady state and transient operation. The device consists of a n-doped amorphous silicon/intrinsic amorphous silicon/p-doped amorphous silicon carbide multilayer structure grown by PECVD on a p-type crystalline silicon wafer doped by a phosphorus diffusion. The steady state behaviour is analysed by an analytical model, which takes into account the absorption coefficients, the diffusion length, the mobility–lifetime product and the layer thicknesses and allows us to obtain quantitative information on the materials inside the device. The linearity of the detector response is also presented. On the basis of the transient response measured by means of a test circuit simulating the row select TFT and the data line capacitance by discrete components, driving of 2-D array of VIPs is discussed.

Recent progress in thallium bromide detectors for X- and γ-ray spectroscopy

Recent progress in the development of room-temperature TlBr detectors is presented. Higher-performance TlBr detectors have been fabricated from high-purity TlBr crystals grown by the traveling molten zone method. The TlBr crystals have exhibited large mobility–lifetime products of 2.6×10 −4 and 3.7×10 −5 cm 2/V for electrons and holes, respectively. An energy resolution of 29.8 keV (5.8%) FWHM has been recorded for 511 keV γ-rays with a TlBr detector at room temperature. Timing properties of the TlBr detectors have been studied by recording the coincidence timing spectra between a TlBr detector and a BaF 2 scintillator coupled to a photomultiplier tube. Timing resolution of 21.3 ns FWHM has been measured for 511 keV positron annihilation γ-rays from a 22Na source. By using a single TlBr detector in current mode, an X-ray CT scanner system of a first-generation type has been developed. The performance of the system has been evaluated by observing tomographic images. Images with good spatial resolution have been obtained by the scanner with the TlBr detector.

Influence of electrical contacts on charge collection profiles in CdZnTe studied by IBIC

The lateral and frontal modes of the nuclear microprobe technique ion beam-induced charge collection (IBICC) were used for imaging the electronic properties of CdZnTe room-temperature detectors. The detectors with different contacts (In–In, In–Au, Au–Au) were bombarded with a scanned 4 MeV proton microbeam. By measuring the detector response at bias voltages of up to several hundred volts, charge collection profiles along the thickness of a detector were calculated for different shaping times . Obvious differences between the various contacts were observed in lateral IBICC efficiency profiles that varied in shape, height and smoothness. No hole collection contribution was observed at apllied voltages for all three configurations of the CZT detector. Electron mobility–lifetime products were calculated from measured IBICC efficiency profiles.

Eluding Metal Contamination in CMOS Front-End Fabrication by Nanocrystal Formation Process

AbstractA technique to form metal nanocrystals on silicon or thin SiO2 film by Rapid Thermal Annealing (RTA) of thin metal film is developed and integrated into standard CMOS processing to make EEPROM devices and improve metal-semiconductor contact resistance. I-V and C-V measurements are carried out on MOSFETs and MOS capacitors containing Au, Ag, Pt, and Si nanocrystals as floating gate for universal mobility and minority carrier lifetime extraction. Mobility around 300 cm2/V-sec and minority carrier lifetime within 0.02 ∼ 0.1 μsec are observed for all cases including the control samples that do not go through the metal nanocrystal formation process, which suggests that the substrate is virtually free from metal contamination. Using this technique, thicker metal film can potentially be achieved as well by stitching thin metal layers on top of the nanocrystals.

Eluding Metal Contamination in CMOS Front-End Fabrication by Nanocrystal Formation Process

AbstractA technique to form metal nanocrystals on silicon or thin SiO2 film by Rapid Thermal Annealing (RTA) of thin metal film is developed and integrated into standard CMOS processing to make EEPROM devices and improve metal-semiconductor contact resistance. I-V and C-V measurements are carried out on MOSFETs and MOS capacitors containing Au, Ag, Pt, and Si nanocrystals as floating gate for universal mobility and minority carrier lifetime extraction. Mobility around 300 cm2/V-sec and minority carrier lifetime within 0.02 ∼ 0.1 μsec are observed for all cases including the control samples that do not go through the metal nanocrystal formation process, which suggests that the substrate is virtually free from metal contamination. Using this technique, thicker metal film can potentially be achieved as well by stitching thin metal layers on top of the nanocrystals.